Darwiniana; Essays and Reviews Pertaining to Darwinism eBook

Views so idealistic as those upon which his “Thoughts
upon Species” [I-2] are grounded, will not harmonize
readily with a doctrine so thoroughly naturalistic
as that of Mr. Darwin. Though it is just possible
that one who regards the kinds of elementary matter,
such as oxygen and hydrogen, and the definite compounds
of these elementary matters, and their compounds again,
in the mineral kingdom, as constituting species, in
the same sense, fundamentally, as that of animal and
vegetable species, might admit an evolution of one
species from another in the latter as well as the former
case.

Between the doctrines of this volume and those of
the other great naturalist whose name adorns the title-page
of this journal, the widest divergence appears.
It is interesting to contrast the two, and, indeed,
is necessary to our purpose; for this contrast brings
out most prominently, and sets in strongest light
and shade, the main features of the theory of the
origination of species by means of Natural Selection.

The ordinary and generally-received view assumes the
independent, specific creation of each kind of plant
and animal in a primitive stock, which reproduces
its like from generation to generation, and so continues
the species. Taking the idea of species from
this perennial succession of essentially similar individuals,
the chain is logically traceable back to a local origin
in a single stock, a single pair, or a single individual,
from which all the individuals composing the species
have proceeded by natural generation. Although
the similarity of progeny to parent is fundamental
in the conception of species, yet the likeness is
by no means absolute; all species vary more or less,
and some vary remarkably—­partly from the
influence of altered circumstances, and partly (and
more really) from unknown constitutional causes which
altered conditions favor rather than originate.
But these variations are supposed to be mere oscillations
from a normal state, and in Nature to be limited if
not transitory; so that the primordial differences
between species and species at their beginning have
not been effaced, nor largely obscured, by blending
through variation. Consequently, whenever two
reputed species are found to blend in Nature through
a series of intermediate forms, community of origin
is inferred, and all the forms, however diverse, are
held to belong to one species. Moreover, since
bisexuality is the rule in Nature (which is practically
carried out, in the long-run, far more generally than
has been suspected), and the heritable qualities of
two distinct individuals are mingled in the offspring,
it is supposed that the general sterility of hybrid
progeny interposes an effectual barrier against the
blending of the original species by crossing.

From this generally-accepted view the well-known theory
of Agassiz and the recent one of Darwin diverge in
exactly opposite directions.

That of Agassiz differs fundamentally from the ordinary
view only in this, that it discards the idea of a
common descent as the real bond of union among the
individuals of a species, and also the idea of a local
origin—­supposing, instead, that each species
originated simultaneously, generally speaking, over
the whole geographical area it now occupies or has
occupied, and in perhaps as many individuals as it
numbered at any subsequent period.